This invention relates to the field of drop testing, and more particularly, to the drop testing of products to understand their behavior during a drop and to determine their reliability after being dropped.
In the prior art, it is known that a product can be drop tested by simply dropping it from a prescribed height. This is known as free drops. However, the angle and location of impact of the product against the floor cannot be precisely controlled, nor can such impacts be reliably repeated. It is also difficult to outfit the product with measuring instruments to obtain information about forces and deformations that occur during the test. This difficulty arises, at least in part, because the product may land a) at an orientation that is incompatible with the measuring instrument, b) on the measuring instrument itself, thereby destroying it, or c) on the connecting wire of the measuring instrument.
Prior art repeatable and controlled drop testing of a product involves the attaching of the product onto a platform which is then dropped in a controlled and repeatable manner, so that when the platform reaches the end of its travel it subjects the product to a shock pulse, the duration and amplitude of which was previously specified. This is known as drop-table type drop testing. However, because the product is on the platform, the product is subjected to the shock pulse as if the product fell flat on the floor and impacted thereon with the face of the product that is against the platform. Moreover, because the product is attached to the platform, the motion of the product is substantially restricted, and so the product does not move in accordance with its natural dynamic.
We have recognized that in the real world products do not necessarily fall neatly on a face, as is simulated by the prior art repeatable and controlled drop testing. Instead, products when they fall tend to land in such a way as to clatter, i.e., they rotate and have multiple impacts with the ground during the course of a single drop, and to chatter. Furthermore, we have recognized that the velocities and forces of secondary impacts may be greater than the force of the initial impact with the ground, and that different parts of the product are subject to different accelerations due to rotation of the product after the initial impact. Thus, conventional drop table type drop testing is insufficient to accurately assess the behavior of a product in a real drop.
Therefore, in accordance with the principles of the invention, drop testing is performed by controlling the position of the product with respect to the drop surface until just before the initial impact, and then the product is allowed to impact like a free body. Advantageously, realistic, controllable. and substantially repeatable free drop testing can be achieved.
In one embodiment of the invention the product is suspended at an angle from a falling structure, e.g., using at least one string or wire, and the suspending material is effectively released just prior to initial impact. By effectively released it is meant actually released, or the effect of the suspending material is essentially negligible, e.g., where the suspending material produces a very low restitutional force when deformed, such as a weak rubber band. The suspension of the object is arranged in such a way that initial impact occurs at the desired point on the product. To this end, depending on the configuration of the object and the number of suspending elements available, it may be necessary that the point of initial impact remain in contact with a platform, which may be part of the structure, that drops along with the product, such as a conventional drop table.
In another embodiment of the invention, the drop surface may be moved with respect to the product rather than the product moving with respect to the surface.